The conversion of concentrated solar power into household and industrial electricity is one of the most promising ways of generating renewable energy in the 21st century. Future solar-thermal power plants will typically be made up of a thermal receiver 1 installed at the top of a tower of several hundred meters height, and hundreds or thousands of heliostats placed on the ground, each including a surface 2 for reflecting incident rays 3 and ensuring the sun is tracked and its radiation concentrated, in a set direction, toward the receiver 1 as shown in FIG. 1a; the heliostats are orientable and the receiver is fixed. Mention may be made of other types of cosmic-radiation concentrators, such as individual concentrators that are directed toward the radiation source (shown in FIG. 1b) but that concentrate the radiation towards a receiver 1 in a direction that varies with the orientation of the concentrator (the assembly made up of the concentrating reflector surface 2 and the receiver 1 being orientable), double-mirror solar furnaces including a field of orientable planar heliostats, each heliostat including a planar surface 2b for reflecting incident rays 3, and a stationary concentrating surface 2a that concentrates the radiation on a fixed receiver 1 (shown in FIG. 1c), such as that of Odeillo in France the dimensions of which are indicated in the figure.
Among the many technological challenges remaining to be addressed feature those of the time and effort devoted to adjusting and inspecting heliostats, or more generally reflective optical surfaces, before the plant is commissioned, and of the need to regularly monitor them in operation.
Each reflective concentrating surface (also referred to by the term “mirror”) is generally, but not necessarily, segmented into a plurality of facets or segments. Each reflective concentrating surface may be parabolic or even spherical or planar, as indicated in the above examples.
The main characteristic optomechanical defects in a reflective surface 2 (or 2a) for concentrating cosmic radiation when said surface is segmented into reflecting facets 21 are shown in FIG. 2; in this figure, two facets Oi and Oj may be seen. These optomechanical defects may essentially be divided into three categories:                Local surface errors δl, which represent the deviation of the reflective facet from an ideal spherical or parabolic shape. These errors may have many origins: manufacturing quality, deformation under mechanical stresses, the effects of the environment, etc. They are generally very variable from one facet to the next.        Errors δn the orientation of the facets with respect to one another; defects in X-, Y- and Z-axial positions have a negligible influence on the concentration factor.        Lastly, the error δp in the overall aim of the heliostat or concentrator, this error having the effect of adding an average plane that is inclined with respect to all of the facets.        
In the case of a concentrating surface that is not segmented into facets, there is no reason for there to be any errors δr in the orientation of the facets with respect to one another; any optomechanical defects are then due to local surface errors δl and the error δp in overall aim.
Once any optomechanical defects have been identified, the adjustment consists in correcting the shape of the mirrors 2, typically by means of mechanical actuators located therebehind and allowing, depending on their design, the orientation (attitude), average curvature or higher-order defects of the mirrors to be corrected. However, experience acquired with existing solar plants (for example, in France, the THEMIS plant in Targasonne or the 1000 kW solar furnace in Odeillo) suggest that these operations for measuring errors and making adjustments will require several months or even years if current techniques are applied to an industrial scale plant of 10 megawatts or more. Furthermore, these operations sometimes require the focal point of the plant to be occupied, thus decreasing boiler uptime.
These optomechanical defects are measured either by taking measurements in the laboratory, the final shape that the facets will have in operation then not being precisely known, or by taking measurements in the field.
Most current techniques, such as optimization of the flux collected by a detector located at the focal point of the plant, or remote observation of a target in the focal plane, disrupt the operation of the solar plant: specifically, these methods involve obstructing the access of the solar rays to the boiler (=the thermal receiver). These techniques are based on a measurement of luminance in the target plane of the receiver (flux densities), from which overall conclusions are drawn on the quality of the reflective surfaces, but they in particular do not allow any local surface errors δl to be determined.
Another solution consists in using a deflectometry technique (observation of a grid or one-dimensional array of fringes through the reflective surfaces) but this technique does not work with apparatuses of the type described with reference to FIGS. 1b and 1c, and the configuration of the inspection, carried out in the laboratory for example, or with a pair of non-conjugated points, is very different from that of the end-use. This technique furthermore requires image processing that is complex or even impossible.
What are called “backward gazing” methods, which consist in placing a detector in the middle of the target plane and from there directly observing images of the radiation source, or luminance distributions on the reflecting surface, are also known, as described in the publication by F. Hénault and C. Royère: “Solar radiation focusing: analysis and determination of reflecting facets point spread functions and alignment errors”, J. Optics 1989, vol 20, n°5, pp. 225-240. However, such methods do not allow a quantitative measurement of local surface errors δl and provide only a crude measurement of the errors δr in the orientation of the facets with respect to one another.
Therefore, there remains to this day a need for an apparatus the reflective surfaces of which can be inspected without disrupting operation of the apparatus and in less time.
The principle of the invention consists in observing, from a plurality of observation points located on the target surface of a working apparatus for concentrating cosmic radiation, the distributions of luminance visible on the surface of the reflecting surfaces, and in deducing therefrom quantitative information on the local surface errors, aiming errors and possibly orientation errors thereof.